Mobility of the pubic symphysis. In vivo measurements with an electromechanic method and a roentgen stereophotogrammetric method.

Mobility of the pubic symphysis can be measured in vivo by two new methods. One is electromechanic (EM), with steel pins inserted into the bone. The other is roentgen stereophotogrammetric analysis (RSA), with tantalum bone markers implanted into the hip bones. With the first method, movements are directly registered with resistive transducers mounted on the pins. With the second method, movements are deduced with kinematic analysis from roentgen stereophotogrammetric measurements of the positions of the bone markers. Both methods have high precision, about 0.1 mm for translations and 0.1 degrees for rotations. The methods were used in two young volunteers, a man and a woman. In provocative tests with identical loading (alternating, active straight-leg raising in the supine position and standing on alternating left or right leg) there was a good correlation between the two methods. The measured values, i.e., translations of up to 2 mm and rotations of up to 3 degrees, form a basis for more comprehensive studies of normal and pathologic mobility of the pubic symphysis.